Magnetic reed switches comprise two reeds made of magnetic material so that when magnetic field is applied the two reeds either contact each other or separate from each other to make or break a contact. Thus, when a permanent bar magnet passes a reed switch the contact between the reeds is either made or broken.
Reed switches are used in level measuring devices wherein a float with a magnet therein moves up and down adjacent a reed switch in a fixed position.
One example of utilizing a reed switch in a level measuring device is shown in my U.S. Pat. No. 5,299,456 entitled "Electronic Dipstick for Indicating the Oil Level of an Engine". Another use of reed switches is shown in U.S. Pat. No. 4,627,283 to Nishida et al. The latter patent illustrates multiple reed switches mounted at an angle in a vertical plane parallel to the movement of a float containing a permanent magnet. The reason for the angle mounting is to position the reed switches as close together as possible so that two switches are actuated simultaneously.
One problem that has always existed with reed switches is what is referred to as "multiple switching". This occurs when the permanent magnet moves in a path which is substantially parallel to a switching line of a reed switch. It does not occur if the magnet moves laterally away or towards the reed switch. In this latter situation there is only a single switching point, but the on/off distance is larger than that with a substantially parallel movement, providing a less accurate device which is seldom used in level sensing or position sensing applications.
The magnetic field from a permanent magnet moving in a path substantially parallel to the switching line of a reed switch causes three possible switching positions when the magnet is mounted with the magnetic poles substantially parallel to the reed switch. In order to avoid this, most reed switches in level sensing or position sensing applications have only a small movement between magnet and switch so that only one of the switching positions is activated.
In order to overcome the multiple switching without having to limit the movement of the permanent magnet, shielding is used. However, magnetic shielding is a costly and complicated method.